It is known in the art to heat aggregates to 150-170° C. to remove water and mix with hot bitumen (asphalt) at 135-150° C. in the mixing chambers of a drum mix plant or a pug mill batch mix plant or a dual mixer. Additives can be added to the aggregate or bitumen to improve bonding and various physical performance parameters of the final Hot Mix Asphalt (HMA).
The prepared HMA is compacted for surface paving at around 120-135° C. For the past two decades, many technologies have evolved to prepare the asphalt/bitumen mix at lower temperatures. For instance, U.S. Pat. No. 7,297,204 is directed to mixing at a lower temperature and to ensure the workability (flow) and compact-ability at a reduced temperature (e.g., 15-35° C. lower than customary) at the paving stage.
Additional efforts have been tried to expand the bitumen by water injection of 1-4% on the bitumen weight, (24 Mar. 2008, Warm Mix Asphalt: Best Practices, NAP A 53rd Annual Meeting—Brian D. Prowell), above 100° C. Addition of water containing zeolite or fillers and water-in-oil emulsion in collar section in the mixing zone are also known in the art.
However, the water injection or zeolite or filler containing water has a disadvantage that the stability of foamed bitumen is poor. In particular, the formed bubbles are large and break easily. As a result, expansion of the bitumen surface and its stabilization during mechanical mixing is very poor resulting in only marginal improvement in coating and workability compared to normal hot mix. Furthermore, it has been noted that the presence of water results in lower compressive strength and poor tensile strength ratio (TSR) for Marshall Mix Design or Super Pave Mix Design.
Waxes are known to be added to improve lubrication effect for improved workability at lower temperatures. Further, combination of emulsifiers and additives has been added to improve wetting & lubrication to achieve the objective of 15-35° C. lower temperature Asphalt mix preparation and compaction.
However, the addition of waxes and chemical additives like surfactants for improving wetting and processability with compaction at lower temperatures also lowers the tensile strength and undesirably increases rutting phenomenon.
The lower temperature of mixing results in lowering the oxidation of bitumen during processing and storage. The lowering of the oxidation of bitumen results in less hardening of bitumen and ultimately results in undesirably softer pavements. As such, the concerns of rutting and deformation still need to be addressed in immediate use when open to traffic.
Also, during snow fall in winter months, salt mixtures of sodium chloride, magnesium chloride, or calcium chloride are sprinkled on asphalt pavements. These salts in wet condition cause significant damage to asphalt pavements.
As such, there remains a need for a way to reduce coating/wetting time of HMA at a lower temperature at reduced effort (resistance to mixing) to prepare “WARM MIX ASPHALT” (WMA) which will have high strength, improved stiffness, reduced moisture distress and improved stripping resistance in water, particularly salt water. Additionally, there remains a need to improve the processability or workability and improve compaction at lower temperatures (e.g., by 15-25° C. lower than standard or industry recognized compaction temperatures). Further, there remains a need to improve the strength of WMA and resistance to moisture distress.